The present invention relates generally to the packaging of electronic components. More particularly, the present invention relates to methods for making bond wire pressure sensor die packages.
FIG. 1A illustrates a pressure sensor die 10 suitable for use with the present invention. In one embodiment, pressure sensor die 10 is a piezoresistive pressure sensor. Piezoresistive pressure sensors are discussed, and one method for making piezoresistive pressure sensors is disclosed, in U.S. Pat. No. 5,719,069, entitled xe2x80x9cONE-CHIP INTEGRATED SENSOR PROCESSxe2x80x9d, issued Feb. 17, 1998 to Sparks, which is incorporated, in its entirety, by reference herein. Another type of pressure sensor is disclosed in U.S. Pat. No. 3,748,571, entitled xe2x80x9cPRESSURE SENSITIVE TRANSDUCERS EMPLOYING CAPACITIVE AND RESISTIVE VARIATIONSxe2x80x9d, issued Jul. 24, 1973 to Kurtz, which is also incorporated, in its entirety, by reference herein.
Pressure sensors and pressure sensor die assemblies are well known to those of skill in the art and come in a large variety of sizes and configurations. Consequently, while one embodiment of a pressure sensor die 10 is discussed below, it will be recognized by those of skill in the art that numerous other types of pressure sensors will work equally well with the present invention.
Referring back to FIG. 1A, pressure sensor die 10 includes a pressure sensitive micro-machine element 54 which is a pressure sensing membrane composed of a portion of the epitaxial silicon layer 16. Pressure sensor die 10 also includes a plurality of piezoresistors 14 formed in epitaxial silicon layer 16. Piezoresistors 14 serve as sensing elements for micro-machine element 54.
Pressure sensor die 10 is formed by bonding a Substrate 18 to a glass or silicon wafer 20. Substrate 18 includes a cavity 22 such that when substrate 18 is bonded to wafer 20, wafer 20 seals cavity 22. Cavity 22 is positioned directly below micro-machine element 54. In one embodiment, wafer 20 hermetically seals cavity 22 so that pressure sensor die 10 is an absolute pressure sensor.
FIG. 1B shows another embodiment of a pressure sensor die 10B, which includes a die hole 60 through wafer 20 to vent cavity 22 making pressure sensor die 10B a differential pressure sensor. As discussed in more detail below, the method and structure of the present invention works equally well with both absolute pressure sensors, such as pressure sensor die 10, and differential pressure sensors, such as pressure sensor die 10B.
Pressure sensor die 10 or 10B is typically used to monitor the pressure of an external fluid, i.e., a gas or liquid, by placing first or outer surface 56 of pressure sensor die 10 or pressure sensor die 10B, including micro-machine element 54, in contact with the external fluid. During normal operation of pressure sensor die 10 or 10B, micro-machine element 54 flexes in response to pressure on first or outer surface 56.
This flex is sensed by piezoresistors 14 and processed to determine the pressure exerted on first or outer surface 56 by the external fluid, i.e., the pressure of the liquid or gas.
The structure and operation of pressure sensors, such as pressure sensor die 10 and pressure sensor die 10B, is well known to those of skill in the art. Consequently, a more detailed discussion of the structure and operation of pressure sensor die 10 and pressure sensor die 10B is omitted here to avoid detracting from the present invention. However, it is worth noting here that, as discussed above, in order for pressure sensor die 10 or pressure sensor die 10B to function, first or outer surface 56, including micro-machine element 54 must be flexibly coupled to the surrounding environment and cannot be shielded from that environment by interposing layers of packaging material such as plastics or epoxies.
FIG. 1C is an enlarged cross-sectional view of a pressure sensor sub-assembly 100 including a pressure sensor die 110 mounted on a substrate 102 in die attach region 131. Like pressure sensor die 10 of FIG. 1A, pressure sensor die 110 (FIG. 1C) includes a pressure sensitive micro-machine element 154 composed of a portion of the epitaxial silicon layer 116. Like pressure sensor die 10 discussed above, pressure sensor die 110 is formed by bonding a substrate 118 to a glass or silicon wafer 120. Substrate 118 includes a cavity 122 such that when substrate 118 is bonded to wafer 120, wafer 120 seals cavity 122. Cavity 122 is positioned directly below micro-machine element 154.
Pressure sensor die 110 is attached to a first surface 111 (a die attach surface) of substrate 102 in die attach region 131 using any one of several well-known adhesives 104. Substrate 102 is typically a printed circuit board (PCB). In one embodiment, electrically conductive contacts or pads 106 on first or outer surface 130 of pressure sensor die 110 are connected with electrically conductive bond wires 103 to electrically conductive traces 112 and/or electrically conductive regions (not shown) formed on first surface 111 of substrate 102. Electrically conductive vias 114 are formed through substrate 102, from traces 112 and/or regions on first surface 111 to a second surface (the mounting surface) 140 of substrate 102 which is opposite first surface 111. Electrically conductive traces 113 formed on second surface 140 of substrate 102 extend to electrically conductive contact or pads 115 formed on second surface 140 of substrate 102. Electrically conductive contacts 115 are used to connect substrate 102 and pressure sensor die 110 to a larger system, such as a mother board (not shown), using well known methods such as solder balls, pins, leadless carrier chip (LCC) contacts or other surface mounts.
In accordance with the present invention, a plurality of pressure sensor die packages are fabricated simultaneously, in an array, to minimize the cost associated with each individual pressure sensor die package.
In one embodiment of the invention, a plurality of pressure sensor dice are attached to an array of pressure sensor die sites located on a substrate. The pressure sensor dice are then electrically connected to the pressure sensor die sites using, in one embodiment, standard wire bond techniques.
The resulting array of pressure sensor die sub-assemblies is then molded, in one embodiment of the invention, using a mold tool that closes on three sides of the substrate so that a cavity is formed that is open on the fourth side. To this end, a first portion of the molding tool has a plurality of insert pins that close on the outer surface of each pressure sensor die.
As a result, using the method of the invention, a portion of the outer surface of the micro-machine element of each pressure sensor die is left exposed at the bottom of a cavity in the molding encapsulant. After molding, the exposed outer surface of the micro-machine element is covered with a pressure coupling gel that is applied in the shallow cavity. The coupling gel protects micro-machine elements from the environment, yet is compressible and is capable of coupling pressure from the external environment to the micro-machine elements.
The resulting array of packaged pressure sensor dice are then sigulated using well known sawing or laser techniques or by snapping a specially formed snap array.
In another embodiment of the invention, a hole is formed in the array substrate and the pressure sensor substrate to accommodate a differential pressure sensor die.
In another embodiment of the invention, a custom substrate is formed with a plurality of holes formed, one each, at pressure sensor die mounting sites. A plurality of pressure sensor dice are then attached to pressure sensor die mounting sites located on the substrate. The pressure sensor dice are then electrically connected to the pressure sensor sites using, in this embodiment, standard flip-chip techniques.
The resulting array of pressure sensor die sub-assemblies is then molded, using a mold tool that closes on the substrate and is filled with encapsulant.
Using this embodiment of the method of the invention, a portion of the outer surface of the micro-machine element of each pressure sensor die is left exposed at the bottom of the holes in the substrate. After molding, the exposed outer surface of the micro-machine element is covered with a pressure coupling gel applied in the hole in the substrate. The resulting array of packaged pressure sensors are then sigulated using well known sawing or laser techniques or by snapping a specially formed snap array.
In another embodiment of the invention, a cavity is formed in the encapsulant and the pressure sensor substrate to accommodate a differential pressure sensor die.
In particular, one embodiment of the method of the invention includes: providing a pressure sensor die, the pressure sensor die having a pressure sensor die first surface and a pressure sensor die second surface, opposite the pressure sensor die first surface; providing a substrate, the substrate having a substrate first surface and a substrate second surface, opposite the substrate first surface, the substrate first surface having a die attach region; attaching the pressure sensor die second surface to the substrate first surface in the die attach region; and applying encapsulant to the substrate first surface and the pressure sensor die, the encapsulant having an outer surface, the encapsulant being applied such that the encapsulant forms a cavity over a first region of the pressure sensor die first surface, the cavity having side surfaces and a bottom surface wherein the first region of the pressure sensor die first surface is the bottom surface of the cavity and the first region of the pressure sensor die first surface is not covered by the encapsulant.
One embodiment of the method of the present invention also includes filling at least a portion of the cavity in the encapsulant with coupling gel such that the coupling gel covers the first region of the first surface of the pressure sensor die.
These and other features and advantages of the present invention will be more readily apparent from the detailed description set forth below taken in conjunction with the accompanying drawings.